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Thermoplastic composites that have been reinforced by thermotropic liquid crystalline (TLCP) fibrils in the microscale and by nanoparticles in the nanoscale are defined as multi-scale wholly thermoplastic composites (WTCs). Multi-scale WTCs have been proposed as lightweight replacements with high performance for some traditional glass fiber (GF) and carbon fiber (CF) reinforced composites materials in various applications. TLCPs are known for performing mechanical properties similar to those of the lower end of CF but significantly better than those of GF. To enhance the mechanical properties of TLPC reinforced WTCs, carbon nanotubes (CNTs) are considered being used as a secondary enhancement in WTCs. CNTs have gathered significant interest in the last 30 years because of their high aspect ratio, high mechanical properties, and other high-performance properties. The focus of this work is on investigating the processing conditions of generating in situ injection-molded multi-scale WTCs, then extending the technology to dual-extrusion and fused filament fabrication (FFF) and obtain high-performance multi-scale WTC products. This dissertation initially focused on investigating the processing conditions, in particular mixing histories and processing temperature profiles, of generating in situ injection-molded multi-scale WTCs, which consist of a representative TLCP, scCO2 aided exfoliated CNTs, and the thermoplastic matrix polyamide 6 (PA 6). The supercooling behavior of the TLCP and thermal stability of PA 6 are studied by applying the rheological methods of small amplitude oscillatory shear (SAOS). Multiple mixing histories with CNTs and processing temperature profiles are analyzed based on the criterion of maximizing tensile properties of multi-scale WTCs and minimizing thermal degradation of the matrix. Under the optimum processing conditions, the in situ injection-molded multi-scale WTCs exhibit a 26% and 34% tensile modulus and strength enhancement, compared to the in situ injection-molded WTCs with no CNTs. Scanning electron micrograph (SEM) images were used to understand the enhancement. The second part of this work is to extend the scCO2 aided in situ multi-scale WTCs processing technology to dual-extrusion and FFF. Multi-scale WTC filaments, which consists of TLCP, CNTs, and polyamide copolymer (PAc), are generated by dual-extrusion, and 3D printed into rectangular specimens in FFF. The 1 wt% CNTs reinforced multi-scale WTC filaments generated by the means of dual-extrusion exhibit 225% and 80% improvement in tensile modulus and strength, respectively, compared to the WTC filaments with no CNTs. In FFF, 40 wt% TLCP/1 wt% CNT/PAc 3D printed specimens with filament laid in longitudinal direction exhibited excellent tensile modulus and strength of 38.92 GPa and 127.16 MPa, respectively. The well-dispersed exfoliated CNTs show high alignment with TLCP microfibrils in the multi-scale WTC filaments and their laid-down specimens, which causes the significant tensile modulus enhancement. Bridging elements are discovered between TLCP fibrils and PAc matrix to improve interfacial adhesion, which is attributed to the well-dispersed exfoliated CNTs. Finally, the significant improvements in tensile properties attributed to scCO2 aided exfoliated CNTs in WTCs are verified on the multi-scale WTCs based on polypropylene (PP). Moreover, additional tensile properties improvements for exfoliated CNTs reinforced multi-scale WTCs are obtained with the use of maleic anhydride grafted polypropylene (MAPP). With 1 wt% CNTs and 16 wt% MAPP dual reinforcement, 20 wt% TLCP reinforced WTCs based on polypropylene (PP) exhibit 265%, 274%, and 182% improvement in the tensile modulus of the filaments, laid up specimens in the concentric pattern and laid up specimens in ±45° rectilinear pattern, respectively. The dual reinforcement also improves the tensile strength of 20 wt% TLCP reinforced WTC filaments by up to 73%. The high alignment between TLCP fibrils and CNTs are confirmed in the multi-scale WTCs based on PP. Besides the bridging elements attributed to CNTs found in the second part of this work, SEM images show that CNTs are partially trapped in TLCP fibrils. / Doctor of Philosophy / Considering the need for environmentally friendly materials, novel thermoplastic composites with high mechanical performance, lightweight, and potentially high recyclability properties were generated in this work. Two types of thermoplastic matrices, polyamide (PA or nylon) and polypropylene (PP) were reinforced with carbon nanotubes (CNTs) and rigid chain polymers known as thermotropic liquid crystalline polymers (TLCPs). CNTs are known for their high mechanical properties and high aspect ratio, which are helpful to reinforce thermoplastic composite materials. During injection molding and the dual-extrusion processes, TLCPs deform into almost continuous microfibrils and reinforce the thermoplastic matrices. Instead of using traditional glass fibers or carbon fibers to reinforce thermoplastics, TLCP reinforced thermoplastic composites, which are defined as wholly thermoplastic composites (WTCs), can retain their mechanical properties during the recycling process such as in injection molding and have better performance during the lay-down process in fused filament fabrication. The goal of this work was to generate CNTs reinforced WTCs for use in injection molding and fused filament fabrication with high mechanical performance. In the injection molding process for generating CNTs reinforced WTC end-gated plaques, it was determined that the optimum thermal mixing histories for the CNTs could be identified by the inspections of the tensile property measurements and scanning electron microscopy (SEM). With the obtained optimum thermal mixing histories with CNTs, CNTs reinforced WTC filaments were generated by dual extrusion technology and used in fused filament fabrication. With 1 wt% addition of CNTs, the tensile properties of WTCs were significantly enhanced in both the filament materials and the laid-down parts. Especially, the CNT reinforced WTC filaments based on nylon matrices exhibited competitive tensile moduli to long carbon fiber reinforced nylon composite filaments, which was also competitive to the properties of aluminum alloys. In addition, the laid-down parts of CNTs reinforced WTC based on PP presented further tensile strength improvement due to the improved interfacial adhesion between the laid-down filaments and between layers, which was attributed to the addition of maleic anhydride grafted polypropylene.

Polymer bonded explosives (PBXs) are composites comprised of energetic crystals with a very high energy density surrounded by a polymer binder. The formation of hotspots within polymer bonded explosives can lead to the thermal decomposition and initiation of the energetic material. A frictional heating model is applied at the mesoscale to assess the potential for the formation of hotspots under low velocity impact loadings. Monitoring of the formation and growth of damage at the mesoscale is considered through the inclusion of a piezoresistive carbon nanotube network within the energetic binder providing embedded strain and damage sensing. A coupled multiphysics thermo-electro-mechanical peridynamics framework is developed to perform computational simulations on an energetic material microstructure subject to these low velocity impact loads. With increase in impact energy, the model predicts larger amounts of sensing and damage thereby supporting the use of carbon nanotubes to assess damage growth and subsequent formation of hotspots. The framework is also applied to assess the combined effects of thermal loading due to prescribed hotspots with inertial effects due to low velocity impact loading. It has been found that the present model is able to detect the presence of hotspot dominated regions within the energetic material through the piezoresistive sensing mechanism. The influence of prescribed hotspots on the thermo-electro-mechanical response of the energetic material under a combination of thermal and inertial loading was observed to dominate the lower velocity impact response via thermal shock damage. In contrast, the higher velocity impact energies demonstrated an inertially dominated damage response. Quantifying the piezoresistive effect derived from embedding carbon nanotubes in polymers remains a challenge since these nanocomposites exhibit significant variation in their electro-mechanical properties depending upon factors such as CNT volume fraction, CNT dispersion, CNT alignment and properties of the polymer. Of interest is electrical percolation where the electrical conductivity of the CNT/polymer nanocomposite increases through orders of magnitude with increase in CNT volume fraction. Estimates and distributions for the electrical conductivity and piezoresistive coefficients of the CNT/polymer nanocomposite are obtained and analyzed with increasing CNT volume fraction and varying barrier potential, which is a parameter that controls the extent of electron tunneling. The effect of CNT alignment is analyzed by comparing the electro-mechanical properties in the alignment direction versus the transverse direction for different orientation conditions. Estimates of piezoresistive coefficients are converted into gage factors and compared with experimental sources in literature. The methodology for this work uses automated scripts which are used in conjunction with high performance computing to generate several 5 μm ×5 μm realizations for different CNT volume fractions. These realizations are then analyzed using finite elements to obtain volume averaged effective values, which are then subsequently used to generate measures of central tendency (estimated mean) and variability (standard deviation, coefficient of variation, skewness and kurtosis) in a descriptive statistical analysis. / Doctor of Philosophy / Carbon nanotubes or CNTs belong to a class of novel materials known as nanomaterials which are materials with length scales on the order of nanometers. CNTs have been widely studied due to their unique mechanical, electrical and thermal properties in comparison to traditional materials such as metals or plastics. Often times, research and applications concerning the use of CNTs involves embedding the CNTs as a filler within a larger composite material system. In the present work, CNTs are considered to be embedded within a polymer. It is known that the electrical properties of such a CNT/polymer composite change in response to the application of a mechanical force. This change in electrical properties is caused due to the presence of CNTs and is used as a means of sensing the mechanical state of the composite, i.e. real time structural monitoring. The extent of the change in electrical properties, also known as sensing, depends upon a number of different factors such as the amount of CNTs used per unit volume of the polymer, how well dispersed or clumped together the CNTs are within the polymer and the type of polymer material used, among other factors. A statistical analysis is performed with several case studies where these factors are varied and the resulting change in the sensing response is monitored. Several important conclusions were made from the statistical analysis with some of the results providing new insights into the sensing behavior of CNT/polymer composites. For example, it was found that a key parameter known as barrier potential, which directly influences the extent of sensing achieved through a mechanism known as electrical tunneling, needs to be several orders of magnitude lower than previously reported values to accurately capture the sensing effects. Key metrics quantifying the extent of sensing from the analysis were found to be in agreement with previously reported experimental results. The significance of such a statistical study lies in the fact that CNT embedded composites are increasingly being proposed and used for sensing applications. The use of CNT embedded polymers to encase explosive crystalline grains such as HMX or RDX is one such example. These explosive grains are used in a number of different civil and military applications such as fuel rocket propellants, industry explosives, military munitions etc. The grains possess extremely high energy densities and are susceptible to undergo violent chemical reaction if a trigger is provided through thermal or mechanical means. As such, the monitoring of the structural state of these explosives is crucial for their safe handling and processing. In this work, the sensing response of a composite material comprising of explosive grains surrounded by polymer material containing CNTs is studied in response to different types of mechanical loads, ranging from mild stimuli to impact. It was found that the sensing mechanism was capable of tracking mechanical damage as well as the resulting temperature increases interior to the composite. In addition to its application to safety and preventative measures, the use of CNTs in this context also provided insight into the mechanisms related to the sudden release of energy in these explosive grains which is of significant interest since this is an active area of research as well.

Conducting vertical nanopillar arrays can serve as three-dimensional nanostructured electrodes with improved performance for electrical recording and electrochemical sensing in bio-electronics applications. However, vertical nanopillar-array electrodes made of inorganic conducting materials by conventional nanofabrication approach still faces challenges in high manufacturing costs, poor scalability, and limited choice of carrier substrates. Here, we report a new type of conducting nanopillar arrays composed of multi-walled carbon nanotubes (MWCNTs) doped polymeric nanocomposites, which are manufactured over the wafer-scale on both rigid and flexible substrates by direct nanoimprinting of perfluoropolyether nanowell-array templates into uncured MWCNT/polymer mixtures. By controlling the MWCNT ratios and the annealing temperatures during the fabrication process, MWCNT/polymer nanopillar arrays can possess outstanding electrical properties with high DC conductivity (~4 S/m) and low AC electrochemical impedance (~104 Ω at 1000 Hz). Moreover, by electrochemical impedance spectroscopy (EIS) measurements and equivalent circuit modeling-analysis, we can decompose the overall impedance of MWCNT/polymer nanopillar arrays in the electrolyte into multiple bulk and interfacial circuit components, and thus can illustrate their different dependence on the MWCNT ratios and the annealing temperatures. In particular, we find that a proper annealing process can significantly reduce the anomalous ion diffusion impedance and improve the impedance properties of MWCNT/polymer nanopillars in the electrolyte. / Master of Science / Conducting vertical nanopillar arrays can serve as three-dimensional nanostructured electrodes with improved performance for electrical recording and electrochemical sensing in nano-bioelectronics applications. However, vertical nanopillar-array electrodes made of inorganic conducting materials by conventional nanofabrication approach still faces challenges in high manufacturing costs, poor scalability, and limited choice of carrier substrates. Compared to conventional nanofabrication approaches, nanoimprint lithography exhibits unique advantages for low-cost scalable manufacturing of nanostructures on both rigid and flexible substrates. Very few studies, however, have been conducted to achieve the scalable nanoimprinting fabrication of conducting nanopillar arrays made of MWCNT/polymer nanocomposites. Here, I'm reporting a new type of conducting nanopillar arrays composed of multi-walled carbon nanotubes (MWCNTs) doped polymeric nanocomposites, which can be manufactured over the wafer-scale on both rigid and flexible substrates by direct nanoimprinting of the perfluoropolyether nanowell-array template into uncured MWCNT/polymer mixtures. We find that the nanoimprinted conducting nanopillar arrays can possess appealing electrical properties with a high DC conductivity (~4 S/m) and a low AC electrochemical impedance (~104 Ω at 1000 Hz) in the physiologically relevant electrolyte solutions (1X PBS). Furthermore, I've conducted a systematic equivalent circuit modeling analysis of measured EIS results to understand the effects of the MWCNT ratios and the annealing temperatures on the impedance of different bulk and interfacial circuit components for MWCNT/polymer nanopillar arrays in the electrolyte.

The current state of the art in structural health monitoring is primarily reliant on sensing deformation of structures at discrete locations using sensors and detecting damage using techniques such as X-ray, microCT, acoustic emission, impedance methods etc., primarily employed at specified intervals of service life. There is a need to develop materials and structures with self-sensing capabilities such that deformation and damage state can be identified in-situ real time. In the current work, the inherent deformation and damage sensing capabilities of carbon nanotube (CNT) based nanocomposites are explored starting from the nanoscale electron hopping mechanism to effective macroscale piezoresistive response through finite elements based computational micromechanics techniques. The evolution of nanoscale conductive electron hopping pathways which leads to nanocomposite piezoresistivity is studied in detail along with its evolution under applied deformations. The nanoscale piezoresistive response is used to evaluate macroscale nanocomposite response by using analytical micromechanics methods. The effective piezoresistive response, obtained in terms of macroscale effective gauge factors, is shown to predict the experimentally obtained gauge factors published in the literature within reasonable tolerance. In addition, the effect of imperfect interface between the CNTs and the polymer matrix on the mechanical and piezoresistive properties is studied using coupled electromechanical cohesive zone modeling. It is observed that the interfacial separation and damage at the nanoscale leads to a larger nanocomposite irreversible piezoresistive response under monotonic and cyclic loading because of interfacial damage accumulation. As a sample application, the CNT-polymer nanocomposites are used as a binding medium for polycrystalline energetic materials where the nanocomposite binder piezoresistivity is exploited to provide inherent deformation and damage sensing. The nanocomposite binder medium is modeled using electromechanical cohesive zones with properties obtained through the Mori-Tanaka method allowing for different local CNT volume fractions and orientations. Finally, the traditional implementation of Material Point Method (MPM) is extended for composite problems with large deformation (e.g. large strain nanocomposite sensors with elastomer matrix) allowing for interfacial discontinuities appropriately. Overall, the current work evaluates nanocomposite piezoresistivity using a multiscale modeling framework and emphasizes through a sample application that nanocomposite piezoresistivity can be exploited for inherent sensing in materials. / Ph. D.

A multiscale model is proposed to study the influence of interfacial interactions at the nanoscale in carbon nanotube(CNT)-polymer nanocomposites on the macroscale bulk elastic material properties. The efficiency of CNT reinforcement in terms of interfacial load transferring is assessed for the non-functionalized and functionalized interfaces between the CNTs and polymer matrix using force field based molecular dynamic simulations at the nanoscale. Polyethylene (PE) as a thermoplastic material is adopted and studied first because of its simplicity. Characterization of the nanoscale load transfer has been done through the identification of representative nanoscale interface elements for unfunctionalized CNT-PE interface models which are studied parametrically in terms of the length of the PE chains, the number of the PE chains and the "grip" position. Referring to the non-functionalized interface, CNTs interact with surrounding polymer only through weakly nonbonded van der Waals (vdW) forces in our study. Once appropriate values of these parameters are deemed to yield sufficiently converged results, the representative interface elements are subjected to normal and sliding mode simulations in order to obtain the force-separation responses at 100K and 300K for unfunctionalized CNT-PE interfaces. To study the functionalization effects, atomistic interface representative elements for functionalized CNT-PE interface are built based on non-functionalized interface models by grafting functional groups between the PE matrix and the graphene sheet. This introduces covalent bonding forces in addition to the non-bonded vdW forces. A modified consistent covalent force field (CVFF) and adaptive intermolecular reactive empirical bond order (AIREBO) potentials, both of which account for bond breaking, are applied to investigate the interfacial characteristic of functionalized CNT-PE interface in terms of the force-separation responses at 100K in both normal opening and sliding mode separations. In these studies, the focus has been on the influence of the functionalization density on the load transfer at the nanoscale interface. As an important engineering material, Epon 862/DETDA epoxy polymer,a thermoset plastic, has also been used as the polymer matrix material in order to see the difference in interfacial load transfer between a network structured polymer and the amorphous entangled structure of the PE matrix. As for thermoset epoxy polymer, emphasis has been put on investigating the effects of the crosslink density of the epoxy network on the interfacial load transfer ability for both non-functionalized and functionalized CNT-Epoxy interface at different temperatures(100K and 300K) and on the functionalization effect influenceing the interfacial interactions at the functionalized CNT-Epoxy interface. Cohesive zone traction-displacement laws are developed based on the force-separation responses obtained from the MD simulations for both non-functionalied and functionalized CNT-PE/epoxy interfaces. Using the cohesive zone laws, the influence of the interface on the effective elastic material properties of the nanocomposites are observed and determined in continuum level models using analytic and computational micromechanics approaches, allowing for the assessment of the improvement in reinforcement efficiency of CNTs due to the functionalization. It is found that the inclusion of the nanoscale interface in place of the perfectly bonded interface results in effective elastic properties which are dependent on the applied strain and temperature in accordance with the interface sensitivity to those effects, and which are significantly diminished from those obtained under the perfect interface assumption for non-functionalized nanocomposites. Better reinforcement efficiency of CNTs are also observed for the nanocomposites with the functionalized interface between CNTs and polymer matrix, which results in large increasing for the effective elastic material properties relative to the non-functionalized nanocomposites with pristine CNTs. Such observations indicates that trough controlling the degree of functionalization, i.e. the number and distribution of covalent bonds between the embedded CNTs and the enveloping polymer, one can tailor to some degree the interfacial load transfer and hence, the effective mechanical properties. The multiscale model developed in this study bridges the atomistic modeling and micromechanics approaches with cohesive zone models, which demonstrates to deepen the understanding of the nanoscale load transfer mechanism at the interface and its effects on the effective mechanical properties of the nanocomposites. It is anticipated that the results can offer insights about how to engineer the interface and improve the design of nanocomposites. / Ph. D.

Im Gegensatz zu herkömmlichen Dehnungsmessstreifen können Carbon nanotube (CNT)-basierte Komposite zusätzlich eine ausgeprägte Druck-abhängigkeit des Widerstandes aufweisen. Deshalb können Drucksensoren aus CNT-Nanokomposite ohne den Einsatz von Verformungskörpern wie z. B. Biegebalken aufgebaut werden. Die möglichen Anwendungsgebiete für diese direkt messenden Sensoren wurden in der vorliegenden Arbeit bei drei industriellen Anwendungen wie z. B. bei Robotergreifarmen gezeigt. Die Zielstellung dieser Arbeit ist die Entwicklung und Charakterisierung eines neuartigen Sensors aus CNT-Nanokomposite. Unter Verwendung von Multi-walled carbon nanotube (MWCNT)-Epoxidharz und interdigitalen Elektroden soll der Sensor auf wenigen Quadratzentimetern Drücke im Megapascal-Bereich und somit Kräfte im Kilonewton-Bereich messen können. Durch die Auswahl geeigneter Werkstoffe und die Modellierung mit der Finite Element Methode wurde der Sensorentwurf durchgeführt sowie der Messbereich abgeschätzt. Die Herstellung der MWCNT-Epoxidharz-Dispersion erfolgte durch mechanische Mischverfahren. Anschließend wurden aus der Dispersion druckempfindliche Schichten mit der Schablonendrucktechnik hergestellt. Dabei wurden die Herstellungs-parameter und besonders der Füllstoffgehalt der MWCNTs variiert, um deren Einflüsse auf das mechanische, thermische und elektrische Verhalten zu untersuchen. Die Charakterisierung der mechanischen Kenngrößen erfolgte mit Zugversuchen und dynamisch-mechanischer Analyse. In den Untersuchungen zeigen die MWCNT-Komposite eine signifikante Steigerung der Zugfestigkeit und eine Erhöhung der Glasübergangstemperatur gegenüber reinem Epoxidharz. Die Abhängigkeiten der Druckempfindlichkeit und der Temperaturempfindlichkeit vom Füllstoffgehalt wurden untersucht. Eine besonders hohe Druckempfindlichkeit, aber auch Temperaturempfind-lichkeit wurde bei Proben mit geringem Füllstoffgehalt (1 wt% und 1,25 wt%) festgestellt. Es ist also wichtig, die richtige Materialkombination für diese Art Sensor zu finden. Die realisierten Sensoren liefern zuverlässige Antwortsignale bei wiederholten Belastungen bis zu einer Belastung von 20 MPa (entspricht 2 kN). Zusätzlich wurde der Temperatureinfluss in einem Bereich von −20 °C bis 50 °C durch eine Wheatstonesche Brückenschaltung kompensiert. Die vorliegende Arbeit zeigt, dass eine zuverlässige Druckmessung mit einer Temperaturmessabweichung von 0,214 MPa/10 K gewährleistet werden kann. / In contrast to conventional metallic strain gauges, carbon nanotube (CNT) composites have an additional pressure sensitivity. Therefore, deformation elements such as bending beam is not needed by using pressure sensors, which are based on CNT nanocomposite. The possible areas of application for these pressure direct measured sensors were showed in three industrial application such as robot gripper. The focus of this work is the development and characterization of a new sensor manufactured from CNT nanocomposite. By using multi-walled carbon nanotube (MWCNT) epoxy and interdigital electrodes the sensor, which has a dimension of few square centimetre, should measure a pressure in mega Pascal range and hence a force in kilo newton range. By the selection of suitable materials and the modelling using finite element method, the sensor design as well as the measurement range were carried out. The MWCNT epoxy dispersion is manufactured by using a mechanical mixing process. Subsequent, the dispersion is used to fabricate pressure sensitive layers by stencil printing methods. Thereby, the fabrication parameters and especially the filler content of the MWCNTs were varied for the mechanical, thermal and electrical investigation. The characterization of the mechanical characteristic values were carried out by using tensile test and dynamic mechanical analysis. The results show a significant increasing of the tensile strength and glass transition temperature in comparison to neat epoxy. Additionally, the influence of the filler content to the pressure and thermal sensitivity were investigated. A highly pressure sensitivity but also a highly thermal sensitivity are obtained for samples with lower filler contents (1 wt% and 1.25 wt%). Therefore, a suitable material combination has to be chosen. The fabricated sensors show reliable response signals by repeated excitations up to 20 MPa (meets to 2 KN). Moreover, the temperature influence ranged from -20 °C to 50 °C was compensated with a Wheatstone bridge. This work demonstrate a direct pressure sensitive sensor with reliable response signals by a thermal deviation of 0.214 MPa/10K.

Carbon nanotubes (CNTs) have been recognized as potential candidates for mainstream device fabrication and technologies. CNTs have become a topic of interest worldwide due to their unique mechanical and electrical properties. In addition, CNTs possess high aspect ratio and low density that make them an important material for various technological applications. The field of carbon nanotube devices is rapidly evolving and attempts have been made to use CNTs in the fabrication of devices like field emitters, gas sensors, flow meters, batteries, CNT-field effect transistors etc. These molecular nanostructures are proposed to be an efficient hydrogen storage material. CNT cylindrical membranes are reported to be used as filters for the elimination of multiple components of heavy hydrocarbons from petroleum and for the filtration of bacterial contaminants of size less than 25 nm from water. Recently, CNT bundles have been proposed to be a good material for low-temperature sensing. CNTs have also been considered as promising filler materials due to extraordinary characteristics mentioned above. Fabrication of nanocomposites using CNTs as reinforcing material has completely renewed the research interest in polymer composites. The conductive and absorptive properties of insulating polymer doped with conducting filler are sensitive to the exposure to gas vapors and hence they can be used in monitoring various gases. The application of fiibre reinforced polymer composites in aeronautic industry are well known due to their high mechanical strength and light weight. Also, the conductive composite materials can be used for electromagnetic shielding. Desired properties in CNT-composites can be attained by adding small amount of CNTs in comparison to traditional filler materials. Due to high aspect ratio and low density of CNTs, percolation threshold in CNT-polymer composites can be achieved at 0.1 vol % as compared to ~16 vol. % in case of carbon particles. The research work ׽0.1 vol. %, as compared to reported in this thesis includes the preparation of multiwall carbon nanotube (MWNTs) and MWNT-polystyrene composites, experimental investigations on low temperature charge transport, and magnetic properties in these systems. This thesis contains 7 chapters. Chapter 1 provides an overview of CNTs and CNT-polymer composites. This chapter briefly describes the methods for synthesizing CNTs and fabricating CNT-polymer composites, charge transport mechanisms in CNTs and composites, and their magnetic properties as well. Chapter 2 deals with the concise introduction of various structural characterization tools and experimental techniques employed in the present work. An adequate knowledge of the strengths and limitations of experimental equipment can help in gathering necessary information about the sample, which helps in studying and interpreting its physical properties correctly. Chapter 3 describes the synthesis of MWNTs and their use as filler material for the fabrication of composites with polystyrene (PS). The characterization results of as-prepared MWNT and composites show that MWNTs possess high aspect ratio (~4000), and are well dispersed in the composite samples (thickness ~50-70 µm). The composite samples are prepared by varying the MWNT concentration from 0.1 to 15 wt %. The as¬fabricated composites are electrically conductive and expected to display novel magnetic properties since MWNTs are embedded with iron (Fe) nanoparticles. Chapter 4 presents the study of charge transport properties of aligned and random MWNTs in the temperature range 300-1.4 K. The low temperature electrical conductivity follows the weak localization (WL) and electron-electron (e-e) interaction model in both samples. The dominance of WL and e-e interaction is further verified by magneto-conductance (MC) measurements in the perpendicular magnetic field up to 11 T at low temperatures. The MC data of these samples consists of both positive and negative contributions, which originates from WL (at lower fields and higher temperatures) and e-e interaction (at higher fields and lower temperatures). Chapter 5 contains the results of charge transport studies in MWNT-PS composite near the percolation threshold (~0.4 wt %) at low temperatures down to 1.4 K. Metallic-like transport behavior is observed in composite sample of 0.4 wt %, which is quite unusual. In general, the usual activated transport is observed for systems near the percolation threshold. The unusual weak temperature dependence of conductivity in MWNT-PS sample at percolation threshold is further verified from the negligible frequency dependence of conductivity, in the temperature range from 300 to 5 K. Chapter 6 accounts on the experimental results of magnetization studies of MWNTs and MWNT-PS composites. The observation of maxima in coercivity and squareness ratio at 1 wt % of Fe-MWNT in a polymer matrix show the dominance of dipolar interactions among the encapsulated Fe-nanorods within MWNTs. The hysteresis loop of 0.1 wt % sample shows anomalous narrowing at low temperatures, which is due to significant contribution from shape anisotropy of Fe-nanorods. Chapter 7 presents brief summary and future perspectives of the research work reported in the thesis.